In recent years, demand for flexible printed circuit boards using a film-like substrate such as a polyester film or a polyimide film is growing. However, there is a problem that a permanent film formed by use of a conventional photo solder resist ink developable with a diluted alkali aqueous solution is poor in flexibility despite good solder heat resistance. Due to this reason, it has been difficult to use the resist ink to the flexible printed circuit board (FPC) requiring the flexibility.
For example, photo solder resist inks available to manufacture the flexible printed circuit boards were proposed in Japanese Patent Early Publication [kokai] Nos. 7-207211, 8-274445 and 9-5997. However, sufficient flexibility has not been achieved yet. In addition, there is a case of lowering the solder heat resistance when trying to improve the flexibility. Thus, the photo solder resist ink having sufficient performance has not been obtained under the present circumstances.